Thrombotic thrombocytopenic purpura (TTP) is a fatal syndrome. Acquired TTP is mainly caused by autoantibodies that inhibit ADAMTS13 enzyme. Plasma exchange is the only effective therapy available to date. In Aim1 of this proposal, we will reengineer and characterize a series of novel recombinant ADAMTS13 variants that exhibit increased specific activity, but are resistant to inhibition by autoantibodies from patients with acquired TTP. The completion of this aim will provide novel insights into the structure-function relationship of ADAMTS13 and change how we treat TTP today. In Aim 2, we propose to determine the antigenic binding epitopes at the amino acid resolution using our novel and groundbreaking deuterium exchange coupled with mass spectrometric analysis approaches. In addition, we will determine the pathogenicity of a panel of inhibitory scFV(s) in murine models of arterial thrombosis and TTP established in the laboratory. The information gained from this study may help our understanding of the molecular mechanisms of acquired TTP and the rational designing of novel recombinant ADAMTS13 variants with desired properties (such as resistance to autoantibody inhibition) for future therapy. Finally, in Aim 3, we will test the hypothesis that ectopic expression of wild-type ADAMTS13 and gain-of-function/antibody-resistant ADAMTS13 variants in platelets would target the therapeutic enzyme directly to sites of injury without being inhibited by circulating anti-ADAMTS13 antibodies. Overall, the information obtained from the completion of the proposed study will provide novel insight into the structure-function relationship of ADAMTS13, help our understandings of the mechanisms of acquired TTP, and provide novel tools for potential therapy of such a fatal TTP syndrome.